1. Field of the Invention
The present invention relates to a capacitance-type electrostatic servo acceleration sensor and, more particularly, to a capacitance-type electrostatic servo acceleration sensor capable of detecting acceleration with high sensitivity and high accuracy. This sensor is especially adapted for detection of acceleration, pressure, displacement, flow rate, and so on.
2. Description of the Related Art
FIG. 3 shows the structure of one conventional capacitance-type electrostatic servo acceleration sensor. This acceleration sensor, generally indicated by reference numeral 500, comprises a capacitance-detecting portion 1, a switched capacitor circuit 53, a pulse-width modulation (PWM) circuit 60, a NOT circuit 61, and a low-pass filter 58. The capacitance-detecting portion 1 has a movable electrode 11 which is capable of being displaced according to the applied acceleration, and a pair of fixed electrodes 12a and 12b mounted on opposite sides of the movable electrode 11. The switched capacitor circuit 53 converts variations in the interelectrode capacitances C1 and C2 in the capacitance-detecting portion 1 into an output voltage Vsc. The pulse-width modulation circuit 60 feeds a servo signal Vsw back to the fixed electrode 12a, the servo signal Vsw being pulse width-modulated according to the output voltage Vsc from the capacitor circuit 53. The NOT circuit 61 inverts the servo signal Vsw to form a servo signal notVsw and supplies this servo signal notVsw back to the fixed electrode 12b. The low-pass filter 58 extracts only low-frequency components of the servo signal Vsw and produces an output signal Vo.
The aforementioned capacitance-type electrostatic servo acceleration sensor 500 operates in the manner described below. When the movable electrode 11 is located midway between the fixed electrodes 12a and 12b, the interelectrode capacitance C1 is equal to the interelectrode capacitance C2. The output voltage Vsc proportional to the capacitance difference (C1-C2) is zero. At this time, the duty factor of the servo signals Vsw and notVsw is 50%, as indicated by the solid line in FIG. 4. Therefore, the electrostatic force that the movable electrode 11 receives from the fixed electrodes 12a and 12b when the servo signal Vsw assumes a level of +Vh is equal in magnitude but opposite in sense to the electrostatic force that the movable electrode 11 receives from the fixed electrodes 12a and 12b when the servo signal Vsw takes a level of 0. Since the frequency (e.g., 10 kHz) of the servo signals Vsw and notVsw is sufficiently higher than the response frequency (e.g., 200 Hz) of the movable electrode 11, the electrostatic force that the movable electrode 11 receives from the fixed electrodes 12a and 12b is accumulated and becomes zero. The movable electrode 11 is kept at rest at the midway location between the fixed electrodes 12a and 12b.
When an acceleration G is applied to the capacitance-detecting portion 1, inertia displaces the movable electrode 11. For example, the movable electrode 11 moves toward the fixed electrode 12a and away from the fixed electrode 12b. This increases the interelectrode capacitance C1 and reduces the interelectrode capacitance C2. Because of these capacitance variations, the output voltage Vsc proportional to the capacitance difference (C1-C2) assumes a value of +E. Correspondingly, as indicated by the dotted line in FIG. 4, the duty factor of the servo signals Vsw and notVsw becomes greater than 50%. Therefore, the electrostatic force that the movable electrode 11 receives from the fixed electrodes 12a and 12b when the servo signal Vsw is at a level of +Vh is different in magnitude from the electrostatic force that the movable electrode 11 receives from the fixed electrodes 12a and 12b when the servo signal Vsw is at a level of 0. These two forces are opposite to each other in sense. The electrostatic force that the movable electrode 11 receives from the fixed electrodes 12a and 12b is accumulated and becomes nonzero. As a result, the movable electrode 11 is repelled by the fixed electrode 12a and attracted towards the fixed electrode 12b. That is, the movable electrode 11 undergoes a force directed oppositely to the direction of displacement. Hence, the movable electrode 11 is returned into the midway point between the fixed electrodes 12a and 12b. Apparently, the movable electrode 11 is hardly displaced. When the acceleration G is applied, the duty factor of the servo signal Vsw changes accordingly as described above. By converting this signal into a dc signal by the low-pass filter 58, the output signal Vo corresponding to the acceleration G is obtained. The conventional capacitance-type electrostatic servo acceleration sensor described above is disclosed, for example, in Japanese Patent Laid-Open No. 337468/1992.
Since the above-described conventional capacitance-type electrostatic servo acceleration sensor 500 performs pulse-width modulation in proportion to the acceleration G, the pulse-width modulation circuit 60 needs an accurate oscillator to enhance the accuracy. This complicates the circuit configuration and increases the cost.
Furthermore, the servo signals Vsw and notVsw are invariably fed back to the fixed electrodes 12a and 12b to constrain the movable electrode 11 by means of the servo-mechanism. Consequently, it is difficult to detect capacitance variations with high sensitivity. That is, it is difficult to detect the acceleration G with high sensitivity.
Moreover, if the offset voltage of an operational amplifier incorporated in the switched capacitor circuit 53 drifts, the potential at the movable electrode 11 varies. The result is that the output voltage Vo contains an error.